A recent glance at the nanostructure of tooth enamel helps to explain the unbelievable resilience of the hardest substance in the human body.
The outer layer of the tooth enamel which surrounds and shields other tissue inside the tooth appears like bone but is actually living tissue. The teeth once developed has no natural capability to self-regrow or repair. Tooth enamel, a very hard substance, harder than steel is produced during the mineralization process and new investigation helps to answer the reason behind its exceptional resilience.
Pupa Gilbert, a biophysicist from the University of Wisconsin-Madison said that every time while chewing, enormous pressure is applied on tooth enamel several hundred times per day. Our enamel being unique manages to do it the whole lifetime, so the question arises how does it prevent any failure?
The answer is the “hidden structure” of tooth enamel which is a microscopic structural arrangement of the nanocrystals forming the outer layer of the teeth. These very minute crystals measuring less than one-thousandth the thickness of a human hair which is also found in the teeth of other animals are made of a kind of calcium apatite known as hydroxyapatite. The work appears in Nature Communications.
Gilbert said that they didn’t have the methods to look at the structure of enamel before this study but they can determine and visualize the color orientation of individual nanocrystals and observe several of them at once with a method called polarisation-dependent imaging contrast (PIC) mapping.
He added that this electron microscopy method reveals the architecture of complex biominerals to the human eye. The scientists found that the hydroxyapatite nanocrystals were not oriented in the way that they had earlier assumed while using the PIC mapping technique. The crystals in enamel are grouped into structures called rods and inter-rods but the team identified misorientations of the crystal between adjacent nanocrystals ranging between 1 and 30 degrees.
The authors wrote in the paper that they have suggested that the misorientation of adjacent enamel nanocrystals provides a toughening mechanism i.e., a transverse crack can propagate across crystal interfaces if all crystals are eco-oriented whereas a crack primarily propagates along with the crystal interfaces if the crystals are misoriented.
The molecular dynamics simulations carried out by the team support the concept as testing this hypothesis in human teeth in real life is not feasible. The cracking is circulated more rapidly through crystal networks that didn’t look like human teeth misorientations (of 1 to 30 degrees) in a computer model configured to simulate the spreading of cracking through the enamel.
The team said that this range of nanocrystal misorientation may portray a sweet spot in crack deflection, selected by the long evolutionary history of the enamel. This sweet spot, crystals that are 1–30° apart may maximize the release of energy along with strengthening. The observed misorientations in enamel play a major mechanical role as crack deflection is an important toughening mechanism. They increase the toughness of enamel at the nanoscale, which is essential to withstand the powerful masticatory forces, nearing 1,000 newtons, repeated several thousand times per day.
Journal Reference: Nature Communications.